Role of DDK in Replication Initiation

DDK (Dbf4-dependent kinase) is a serine/threonine protein kinase conserved from yeast to humans. DDK is composed of two subunits, Cdc7 protein kinase and Dbf4 regulatory subunits in 1:1 stoichiometry. Both the CDC7 and DBF4 genes were discovered in budding yeast from the analysis of conditional mutants that were defective in the initiation of DNA replication. Cdc7 and Dbf4 homologues have been identified in many eukaryotes and are important in DNA replication, indicating the role of DDK was also conserved. This knowledge has been translated medically as oncogenic DDK overexpression is currently a target of therapeutic inhibitors. DDK activity is cell cycle regulated because it is inactive in G1 phase cells due to the absence of the essential Dbf4 protein as a result of APC-dependent proteolysis. It is clear from both genetic and biochemical studies that several subunits of the hexameric MCM2–7 DNA helicase/ATPase are substrates of DDK. In an allosteric model of DDK function, DDK phosphorylates the Mcm4 protein in the misaligned MCM2–7 double hexamer bound to origins of replication to align the important catalytic residues of the enzyme and to load other proteins to form a CMG (Cdc45-MCM-GINS) holoenzyme. To complete the initiation reaction, the loading of several other replication proteins is also needed, which requires ensuing CDK (cyclin-dependent kinase) phosphorylation. DDK has other substrates important for mutagenesis by TLS (translesion synthesis), meiotic recombination, and chromosome cohesion. Because all these processes are chromatin-based, DDK may have evolved to regulate chromatin-bound proteins in DNA metabolism.